When you're playing as zerg against zerg, or as zerg against any race for that matter, by sacraficing about 10 seconds on your 6 zergling rush time, you can ensure that you won't be slowed down afterwards, and if you're playing against zerg, you're rush will be pretty close to invincible. As soon as the map starts, to add speed, select all 4 drones and send each to different mineral patches (if you don't know how to do this, select all 4 and right click on one mineral patch. Then, hold shift and click on one of the unit wireframes on the bottom of the screen to deselect one. Now you'll have 3 selected instead of 4. Next, click on the next mineral patch, and do it again until each is on it's own patch). (and don't forget to scout with your overlord)

Next, a drone. When you get another 50, hatch another, and then another. Then, when you have the third one hatch, have it make a spawning pool. WHILE THE POOL IS COMING IN, hatch another drone, then an overlord, and then another drone. You will be able to afford the 2 drones and the overlord, and the timing is perfect. The overlord will come just a few seconds before the pool finishes, and you will have 150 minerals and 3 larvae to hatch your 6 lings. Don't forget to keep building while you attack. Against terrans, this may not work, because the extra 10 or so seconds this is behind the best 6ling rush will give them time for a bunker if they're good. Try not to attack a terran, or just go for the scv's.

NOW, HERE'S THE REASON THIS IS ALMOST PERFECT AGAINST THE ZERG. Remember the 2 drones you hatched while the pool was coming in? Send them with the zerglings. You'll have enough money when you get to their base to set up 2 sunken colonies. If they also 6ling rush, since yours is a little slower, their units will have left their base already, and they'll pull them back frantically to defend when they see 2 sunken colonies forming and 6 zerglings. Even if they set up defense, use the zerglings to distract any sunkens until yours come in. If they live? Keep up the pressure. They'll fold soon enough, especially if they're battling a sunken or 2. (Against any other race, use the 2 drones for gas or minerals or sunkens to defend rather than attack) Almost guaranteed to work on non-MASTER zerg players, and even some of the best fall to this.

(If you find that you aren't getting enough minerals to hatch the 6 lings, or that you don't have enough for the overlord, try having the 3'rd drone go to minerals as well and the 4'th go to make the spawning pool. With practice though, you'll be able to do it with the 3'rd efficiently)

CreepColony's Opinion: A good strategy against Zerg, but against Terran and Protoss, your standard 6 zergling rush would do.